1. A study has been made of the influence of temperature and humidity upon the growth of the chick embryo using a modern mechanically ventilated incubator. Daily measurements beginning the fourth and continuing through the twentieth day of incubation were made on wet weight, dry weight, total nitrogen and total ash of embryos and their extra-embryonic membranes. A total of 18,360 individual weights and determinations was made during the course of this study.

2. The data on growth were obtained from six separate incubation operations conducted at two institutions over a period of 5 years.

3. The percentages of relative humidity used in this study did not cause a significant difference in the growth of the embryo as measured by wet weight.

4. The dry weight of the embryo was the best of the four measures of growth as observed from the data.

5. The various percentages of relative humidity used in this study did not cause a significant difference in the growth of the embryo as measured by dry weight.

6. There is little if any significant variation in the growth of the embryos at the different humidities when total nitrogen is used as an index.

7. A significant difference was observed on the sixteenth, seventeenth and eighteenth days between the total ash of embryos incubated at the various humidities. The difference obtained showed that an intermediate humidity (62 percent) is to be preferred to the extreme humidities.

8. The inclusion of the extra-embryonic membranes did not show that the different humidities affected the growth of the embryo significantly on the twentieth day.

9. The loss of moisture from the egg is apparently influenced by five factors: Temperature of the incubator, humidity within the egg chamber, circulation of air around the egg, shell texture and the thickness and surface area of the egg shell.

10. The study of the growth of embryos incubated in different types of incubators emphasizes that the type of incubator governs the temperature and humidity at which the machine should be operated. A knowledge of the limits of the various influences is essential if standards for optimum conditions in the incubator are to be established.

11. The dry weight data for the two temperatures studied show clearly that temperature exerts a decided influence on the growth of the chick embryo during the early stages of development.

12. It is evident from this experiment that the upper limits of temperature in a mechanically ventilated incubator of the type used are between 38.3° C. (100.9° F.) and 40.5° C. (104.9° F.).

13. This research indicates that 40.5° C. (104.9° F.) in a mechanically ventilated incubator is equivalent to or higher in temperature than 41.6° C. (106.9° F.) in a " sectional" type incubator.

14. Growth results were practically equal when the mechanically ventilated incubator was operated at a temperature 2° F. lower than the " sectional" type of machine.